1. Field of Invention
The present invention relates to the application of coatings to welded joints between sections of pipe, usually as the pipe is being laid.
2. Description of Prior Art
Pipe for petroleum product, particularly for transporting the product through bodies of water, is typically coated with a material, such as several inches of concrete, for protective purposes. However, a portion at each end, or stub, of a section of pipe was left uncoated so that the sections could be welded together as the pipeline was being formed and laid.
Several methods have been proposed to coat the uncovered stubs once the weld had been made and inspected and an anti-corrosive coating applied. A concrete outer coating could be applied, but weight and storage problems were presented, particularly for pipe being laid from vessels in bodies of water. Another approach proposed was to encase the exposed stub portions in a rigid metal mold to form a cavity into which an expanding foam was injected. A final attempt was to form a similar type of annular cavity about the exposed stub portions with an annular sling formed from a suitable synthetic resin and reinforced by a chainlike structure. The pipe sections being connected into the pipeline passed through the annular sling as the stubs were brought into position for application of the foam coating. As a consequence, the sling could be damaged from contact with the pipe. Further, the sling was suspended from two movable clamps or jaw members which included structure to control the tension in the belt and reinforcing jacket. In order for the coated pipe to pass through this apparatus without damaging the sling, the sling had to be kept relatively loosely hung from the jaw members to be kept spaced from the pipe coating. When the loose sling was later tightened to be brought into position for coating, it was difficult to align and uniformly tension. No provision was made to keep the sling aligned in parallel with the pipe. Also, in certain situations, the expanding foam separated the belt ends and left a ridge or irregular surface on the coating. Another problem was that the foam and sling tended to adhere, sometimes requiring four or more people to pull the sling away from the coated pipe once the foam had set. The structure used in this approach was also suspended from a cable by an overhead frame, causing it to be cumbersome and unwieldy, as well as unstable in rough seas, and further occupying needed space on pipe-laying barges.